1. Technical Field of the Invention
This invention relates to phase lock loops (PLLs). More specifically, this invention relates to linearizing the gain of a varactor-tuned voltage controlled oscillator (VCO) with respect to its input voltage for use in optimizing a PLL.
2. Discussion of the Related Technology
All varactor-tuned VCO circuits have a non-linear transfer function characteristic in which the VCO gain changes with the DC input level. FIG. 1 shows a varactor-tuned VCO transfer function and gain definition. The transfer function 101 shows that as the input voltage increases, the VCO gain 102 increases non-linearly. This non-linearity creates difficulties when designing a PLL, because the entire PLL loop gain, bandwidth, and damping response varies with respect to the oscillator frequency. To control the overall gain of a PLL, it is necessary to compensate for the non-linearities of the varactor diode in the VCO.
Traditionally, compensation is achieved by making the varactor diode a small component of the total capacitance of the VCO using a resonant inductance-capacitance (LC) circuit connected to oscillator active circuitry. The design of a resonant LC circuit tuned VCO normally includes a varactor diode, used to adjust the effective capacitance of the tuned circuit when voltage across the diode is varied. To achieve a wide range of frequency variation requires a wide range of capacitance variation. Further, the resonant frequency of an LC resonator varies as the square root of the variation in capacitance. Thus, varactor diodes used previously have been designed with an extremely large range of capacitance variation with a respectively large variation in applied voltage, typically up to thirty volts.
Effectively, the gain of a VCO is the change in the output frequency with respect to the change in the input voltage. Thus, it is desirable to run a VCO at very low gain levels. Yet in some instances, a VCO with the capability of swinging over very wide frequency ranges will be desired. Very wide swing, however, is contradictory to low gain in a VCO. Additionally, it is desirable to have a VCO with a very large output frequency range corresponding to a large DC input voltage range. Large range variation creates a problem, however, when there are small input voltage perturbations such as noise. Referring back to FIG. 1, note that in a high voltage situation, any input noise variation will be greatly amplified by the VCO and create a large random variation of the output frequency, otherwise known as phase noise. Phase noise is a critical parameter of all VCO circuits and its control and reduction is usually a major design exercise.
One prior art technique for controlling phase noise is to limit the maximum frequency variation at the VCO output by placing a large capacitance in parallel with the varactor diode. FIG. 2 shows a prior art VCO with reduced gain via a parallel capacitor. Resonant LC circuit 20, attached to oscillator active circuitry 73, has additional capacitor 201 arranged in parallel with varactor diode 704 and inductor 706. This arrangement reduces the resonator capacitance variation, because the varactor diode 704 makes a smaller contribution to the total capacitance of the resonant LC circuit 20.
Similarly, an additional capacitor can be placed in series with a varactor diode for higher frequencies, with a similar variation effect. FIG. 3 shows a prior art VCO with reduced gain via a series capacitor. Resonant LC circuit 30 is connected to oscillator active circuitry 73 to create a VCO. Additional capacitor 301 is placed in series with varactor diode 704 in resonant LC circuit 30. Both of the prior art techniques shown in FIGS. 2 and 3 reduce the VCO gain and ultimately the circuit phase noise.
In the arrangements shown in FIGS. 2 and 3, the maximum frequency range is severely limited. In many PLL systems, however, it is still important to have a VCO that covers the high gain frequency range while maintaining the low phase noise response of the low gain frequency range. In order to remedy this situation, the technique of bandswitching was implemented using either multiple VCOs or VCOs with multiple resonant networks. With multiple VCO bandswitching, several VCOs, each designed to operate in a narrow band, are switched on, one at a time.
FIG. 4 shows a prior art VCO with capacitance bandswitching. Resonant LC circuit 40 contains several additional capacitors 401, 402, 403 that can be switched in via switches 411, 412, 413. For any combination of enabled switches, the VCO covers a narrow frequency range simply by varying the varactor control voltage. If it is necessary to shift the frequency substantially, one or more capacitors 401, 402, 403 can be added or removed via switches 411, 412, 413. The result is that the noise behavior of the low gain VCO is maintained. Both multiple VCOs and multiple resonant networks avoid the problem of linearizing any single network.
Another prior art technique for handling the non-linear VCO gain problem was to build a very large DC compensator network utilizing diodes and resistors to change the voltage driving the varactor diode. FIG. 5 shows a prior art VCO with control voltage compensation. FIG. 6 shows a transfer function of a prior art VCO with control voltage compensation. In FIGS. 5 and 6, a compensator DC input signal 503 is applied to a control voltage compensator 501, which outputs a compensator VCO control voltage 504 with a non-linear compensator transfer function 602. The compensator VCO control voltage 504 enters VCO 502, combines with the non-linear VCO transfer function 101 (from FIG. 1), and provides a combined VCO output 505 that has a linear transfer function 601. This scheme, however, induces excessive noise at the VCO output.